Magnetic recording hard disk drives use a servo-mechanical positioning system to hold the read/write head on the desired data track and to seek from track to track as required to perform read and write operations. Special “servo” information is written in fields in circumferentially-spaced servo sectors in each of the concentric data tracks on each disk surface. The servo pattern is constructed across multiple tracks so that the read-back signal from the head, as it passes over the pattern, can be decoded to yield the radial position of the head. The servo pattern is written onto the disk during manufacturing in a process known as servowriting.
In conventional servowriting the servo pattern is written in multiple passes using the regular write head in conjunction with a specialized servowriter. The servo pattern may also be written using a magnetically printed preliminary pattern followed by a detailed final pattern, by a media-level servowriter (e.g., a stack of 10 disks servowritten with servowriting heads), or by self-servowriting by the disk drive without a specialized servowriter. Each servowriting pass must be precisely aligned circumferentially. Misalignment introduces errors into the servo system. As the density of the tracks in the radial direction and the linear density of the data bits in the circumferential or along-track direction increase it becomes increasingly difficult to precisely align the servo fields circumferentially.
What is needed is a magnetic recording disk having a servo pattern, and a disk drive having a servo decoding system, that are not sensitive to misalignment of the pre-recorded servo fields.